A shame this machine was never released.
I seem to recall the codename was Armadillo . I'm not sure "existing hex-bus interface" is too appropriate...hex-bus didn't exist for the 99/4A and 99/4A expansion peripherals were not compatible with the 99/8. Hex-Bus was ultimately used on the 99/2 (also never released) and the Compact Computer 40 (the cc-40 WAS released). A hex-bus adapter was meant to allow the 4A to use hex bus peripherals but that also wasn't released.
Released Hex bus peripherals include RS232, printer/plotter, and modem. A wafertape (stringy floppy) drive was announced but not reliable enough so wasn't released.
More recent TI programmable BASIC calculators use a port that is signal compatible but not pin-compatible with Hex-Bus.

The basic concepts of the 99/8 were later mostly implemented in the Myarc Geneve, with the same processor and memory capabilities, although no Pascal or Speech roms. Speech was available with a speech adapter card by Rave99, and Pascal was meant to be included in software but was never finished.
The Geneve has better graphics thanks to its TMS9938 (Yamaha 9938) video chip. The 99/8 used the same TMS9918A as the 99/4A and thus lacked 80-column graphics.
The 99/2, 99/8, Geneve, and 4 and 4A models are all emulated thanks to MESS.

Additional information from Ken Gregg :

I was the software design engineer who developed the HEXBUS firmware (effectively, a device driver with some file system functions sitting in ROM) for both the TI 99/8 and the TI HEXBUS Interface (that connected to the TI 99/4A).
The codename of the 99/8 project was indeed "Armadillo," though many of the engineers as well as our source code control system called it "Dillo" for short. The code name for the HEXBUS controller ship in the 99/8 was "OSO", Spanish for "bear." Legend has it that this name was given by the hardware engineers, because of the chip's complexity. My understanding is that exactly 150 TI 99/8 systems were built before the company announced that it would leave the home computer business in October 1983.

Jim Fetzner adds:

There were a few additional HexBUS peripherals that saw the light of day, but which were either never released or only made it to limited release. There was an 80-column printer (limited release) and a 320K floppy drive (at last count there were eight known surviving examples from TI and two additional units that were reverse engineered and built by a hobbyist in Germany). The video processor was a TMS9118, not a TMS9918A (the two chips have similar capabilities). Several 99/8 peripherals exist that used the expansion interface as well: I personally have a round cable Armadillo Interface (designed to connect to V1 motherboards with a 50-pin Centronics-style Interface). This allowed use of the Peripheral Expansion Box from a TI-99/4A. V2 motherboards had a 50-pin card edge slot expansion interface. Signals were the same, but the pin-outs are radically different. I designed a small adapter board to allow connection to the round cable Armadillo Interface, which I shall be testing shortly. I also have a 128K memory expansion card and an RS-232 card designed to operate with the 99/8. The Myarc Personality Card (A SASI hard disk interface) also contained DSR code to allow use with a 99/8. Most other peripheral cards designed for the TI-99/4A also work with it with one exception: memory cards.